The present invention relates to components and methods for making microelectronic assemblies.
Microelectronic components such as semiconductor chips commonly incorporate numerous contacts exposed on a surface of an element as, for example, on the front surface of a semiconductor chip. These contacts must be connected to another as, for example, to contact pads on a circuit board so as to make the electrical connections between a chip and a larger circuit. Complex semiconductor chips incorporate hundreds of contact pads in a small area. The connections between the contacts on the chip should be reliable and should be manufacturable by an economical process. Moreover, the connections should be compact. In certain applications, it is desirable to mount a chip in such a way that the mounted chip occupies an area of the circuit board equal to or only slightly larger than the surface area of the chip itself. Although the contacts on a chip can be directly bonded to contact pads on a circuit board using a technique known as xe2x80x9cflip-chip bonding,xe2x80x9d this approach suffers from some significant drawbacks. The assembly is heated by heat generated within the chip while the assembly is operating. When operation ceases, the assembly cools again. As the assembly is heated and cooled, the chip and substrate expand and contract by different amounts. The contact pads on the substrate tend to move with respect to the contacts of the chip, thus imposing substantial stresses on the solder balls. This can lead to fatigue failures of the connections in service, rendering the assembly unreliable. As disclosed in certain preferred embodiments of U.S. Pat. Nos. 5,148,265; 5,148,266; and 5,455,390, the disclosures of which are hereby incorporated by reference herein, a sheet-like, preferably flexible, connection component is mounted so as to overly a surface of the chip. The terminals on the connection component are connected to the contacts on the chip by flexible leads. The connection component is arranged so that the terminals are moveable with respect to the chip. For example, the connection component may incorporate compliant layers disposed between the terminals and the chip. The assembly of the connection component and chip provides a packaged chip which can be mounted to a circuit board. The terminals can be bonded to contact pads on the circuit board as, for example, by soldering or otherwise metallurgically bonding the terminals to the contact pads of the circuit board. Because some or all of the terminals overlie the surface of the chip, the area occupied by the assembly can be approximately the same as the area of the chip front surface or only slightly larger than such area. Because the terminals are moveable with respect to the chip, differential thermal expansion of the chip and circuit board does not impose large stresses on the bonds between the terminals and the circuit board.
U.S. Pat. No. 5,518,964 (xe2x80x9cthe ""964 patentxe2x80x9d), the disclosure of which is incorporated by reference herein, discloses further improvements. As disclosed in certain preferred embodiments of the ""964 patent, an array of leads can be formed on a first surface of a sheet-like dielectric connection component. Each lead has a fixed or anchor end permanently attached to the connection component and a tip end remote from the anchor end. The anchor ends of the leads typically are connected to terminals on the second side of the sheet-like component. The tip ends of the leads are releasably connected to the connection component. The tip ends of the leads are disposed in positions corresponding to the positions of the contacts on the chip.
The connection component is juxtaposed with the chip, with the lead-bearing first surface of the component facing toward the contact-bearing surface of the chip. The connection component is aligned with the chip so as to align all of the tip ends of the leads with the appropriate terminals, and the tip ends of the leads are bonded to the contacts on the chip. The connection component is then moved through a preselected displacement in a direction away from the contact-bearing surface of the chip, commonly referred to as a xe2x80x9cverticalxe2x80x9d direction, thereby detaching the tip ends of the leads from the connection component and bending the leads towards a vertically-extensive disposition, leaving the leads free to flex. The step of moving the elements vertically away from one another is commonly referred to as xe2x80x9cliftingxe2x80x9d. Preferably, a curable liquid is introduced into the space between the chip and the connection component during or after the lifting step and cured to form a compliant layer between the chip and the connection component. Because the tip ends of the leads are held in position during the alignment and bonding process, all of the tip ends can be bonded in a single operation. This process is particularly well suited for use with chips having a large number of contacts. This process can be used with individual chip or with elements incorporating many chips such as an entire wafer. All of the connections can be made in a single bonding operation and all of the leads can be bent in a single lifting operation. After these operations, the connection component and wafer can be severed to form individual packaged chip units, each including one chip or a few chips and a portion of the connection component associated with the chip or chips in such unit. Such a unit can be connected to a circuit board by bonding the terminals of the connection component to the circuit board, as discussed above. Here again, the terminals on the connection component are movable with respect to the chip so as to alleviate stress due to differential thermal expansion.
Other embodiments disclosed in the ""964 patent include providing the leads initially on the surface of the chip, with the fixed ends of the leads permanently connected to the contacts of the chip and with the tip ends of the leads releasably connected to the chip. The chip is aligned with a structure such as a connection component or circuit panel having contact pads on the surface facing towards the chip. The tip ends are bonded to the contact pads and the two elements are moved away from one another so as to break the tip ends of the leads away from the surface of the chip and bend the leads in the same manner discussed above. In still other embodiments taught in the ""964 patent, elements other than chips can be connected. For example, two connection components can be connected to one another. The leads are provided on one of the connection components, and the tip ends of the leads are bonded to contacts on the other connection component. The two components are moved away from one another so as to detach the tip ends of the leads from the first connection component and bend the leads towards a vertically extensive disposition.
In certain embodiments taught in the ""964 patent, the leads, in their initial, undeformed, planar condition are curved in horizontal directions along the surface of the component or chip carrying the leads. This curvature provides slack in the leads after the leads are bent towards a vertically extensive disposition.
Components and methods as taught in the ""964 patent provide numerous advantages. For example, the ability to make numerous connections reliably in a single series of operations greatly reduces the costs and simplifies the process. The resulting units or packaged chips can have a surface area approximately equal to the surface area of the chip itself. Although the process disclosed in the ""964 patent can be applied to essentially any configuration of contacts on a chip, it is particularly well suited to forming connections with an element having contacts disposed in a two-dimensional array, commonly referred to as an xe2x80x9carea array.xe2x80x9d
Despite these improvements, still further improvement would be desirable. In certain instances, a semiconductor chips or other component has contacts disposed in an area array with small contact-to-contact distances or xe2x80x9cpitch.xe2x80x9d The tip ends of the leads must be disposed on the mating component at correspondingly small center-to-center distances, and the entire lead thus must fit within a small area. For example, in a chip having contacts disposed in a two-dimensional area array with a 300 micron (0.3 mm) center-to-center distance or pitch in both dimensions, the nominal area occupied by the array is only 0.09 mm2 per contact. Because the contacts on the chip have a finite size, the clear area free of contacts is less than the nominal area per contact of the contact array. Each lead must be placed so that the tip end of the lead will be in registration with the desired contact on the chip when the chip and connection component are engaged with one another, but other portions of the lead will be registered with the clear area of the contact array and thus avoid engagement with neighboring contacts. This is necessary to avoid undesired connections between the lead and neighboring contacts. The available area for lead placement is reduced still further by the effects of tolerances. The leads must be placed on the connection component so that there is some finite clearance between the lead and the neighboring contacts when the array of leads is registered with the neighboring contacts, so that undesired connections do not occur even if the leads are slightly misplaced on the connection component or the connection component is slightly misaligned with the chip. The leads also must be placed on the component so that the leads do not touch one another after the leads are bent towards a vertical disposition during the lifting step.
It is desirable to provide leads of sufficient length to accommodate the vertical movement or lifting operation and to minimize stresses in the leads in service. Assemblies of this type having long leads will encounter lower bending stresses in the leads than otherwise comparable assemblies having short leads. Also, the leads typically must have enlarged portions at their anchor ends to provide for secure attachment of the anchor ends to the connection component. The leads commonly have slightly enlarged portions at their terminal ends to provide adequate for bonding to the contacts, and to aid in fabrication of certain releasable attachments to the underlying surface of the component.
All of these factors, taken together, contribute to a need for further improvements in the lead structures of lead-bearing components.
One aspect of the present invention provides components for making microelectronic assemblies comprising. A component according to this aspect of the invention desirably includes a body having a surface and a plurality of nested leads extending over said surface. Each of the nested leads has first and second ends and an elongated main portion extending between said ends. The leads are xe2x80x9cnestedxe2x80x9d in that the main section of each nested lead is curved to encompass and partially the first end of another one of the leads. Thus, the main portions of each nested leads desirably surrounds an interior area on the surface, and the first end of each nested lead is disposed within the interior area encompassed by another one of the nested leads.
The nested leads desirably are arranged in groups or xe2x80x9cunit cellsxe2x80x9d, and the first end of each nested lead desirably is at least partially surrounded by the main portion of another lead in the same group. Most preferably, the nested leads are arranged in pairs, and the first end of each nested lead partially surrounded by the other lead in the same pair. In one preferred arrangement, the each nested lead is generally hook-shaped, with an enlarged disc-like region forming the first end of the lead and a narrow main portion extending in a partial loop from the first end. The two leads of the pair desirably are congruent with one another, but rotated 180xc2x0 with respect to one another. The first ends are offset from one another in direction, arbitrarily referred to as the Y horizontal direction, and lie on a common central axis extending in the Y direction. The second ends of the leads are offset from one another in an X direction transverse to the Y direction, and are disposed remote from the central axis. As further explained below, the preferred components according to this aspect of the invention provide a uniquely compact lead arrangement with leads having main sections of substantial length. Typically, the first end of each lead serves as an anchor end and is fixed to the body of the component, whereas the second end of each lead is a tip end which is releasably attached to the body or movable with respect to the body.
A further aspect of the invention provides methods of making connections. A method according to this aspect of the invention desirably includes the step of providing first and second elements and a plurality of generally planar nested leads connected between these elements. Each nested leads desirably has a first end connected to the first element, a second end connected to the second element and an elongated main section extending between the first and second ends. Here again, the main section of each nested lead desirably is curved to encompass and partially surround the first end of another one of said nested leads. For example, the leads may be provided on a component as discussed above, which serves as the first element. Such a component can be juxtaposed with the second element and the second ends of the leads can be connected to contacts on the second element.
The method according to this aspect of the invention desirably includes a further step, referred to herein as a xe2x80x9clifting stepxe2x80x9d, of moving the first and second elements away from one another with a component of motion in a vertical direction normal to the plane of the leads so as to bend said nested leads into a vertically-extensive disposition.
These and other objects, features and advantages of the invention will be more readily apparent from the detailed description of preferred embodiments set forth below, taken in conjunction with the accompanying drawings.